Effects of Sheep Grazing on Bumblebee Flower Visitation Rate in an Alpine Ecosystem

Effects of sheep grazing on bumblebee flower visitation rate in an alpine ecosystem – no bees were killed during this experiment – A B S T R A C T ….................................................................................................................................................................. A permanent landscape scale grazing experiment was established at Hol, Buskerud, in a lower alpine ecosystem with a recent history of rather low grazing pressure. Since 2002, nine sub-enclosures has been permanently subjected to either of three treatments of sheep grazing pressure: grazing-ceased, grazing-maintained and grazing-increased. By applying a transect based study-design with 15 replicates for each treatment, I investigated how bumblebee abundance responded to the grazing gradient. The effect of grazing treatment on bumblebee flower visitation rate was analysed in a glmmADMB model setting together with a number of other independent variables. The grazing-maintained treatment had the highest flower visit count with 260, while the grazing-ceased treatment had 223, and the grazing- increased treatment had 166. However, the conclusive “best model” disregarded grazing treatment as an explanatory factor, partly due to naturally occurring variation in in real time bumblebee distribution. I conclude by putting my findings into a grazing ecological context. ….................................................................................................................................................................. Table of contents: 1. Introduction 1.1 Historical background and present day context of grazing practices 1.2 The grazing experiment at Hol 1.3 Sheep and their grazing habits 1.4 Vegetational responses to grazing 1.5 Bumblebees 1.6 Aims 2. Method 2.1 Data sampling 2.2 Statistical model 3. Results 4. Discussion 5. Conclusion 6. References 7. Appendix ….................................................................................................................................................................. 1 1. Introduction As a master student to be, and a passionate bumblebee enthusiast, I was delighted when I was given the opportunity to do research on bumblebee assemblages as part of a large scale grazing experiment at the mountains of Hol, Buskerud. In my introduction, I will first present grazing practices in the context of society, and then introduce the grazing experiment at Hol thoroughly, as much of the subsequent text rests upon this information. Thereafter, I will spend time exploring grazing and plant community responses, mainly in alpine settings, thus putting my work thoroughly into a grazing ecological context. I round this section off with a look at alpine bumblebees before I present my aims. 1.1 Historical background and present day context of grazing practices Livestock grazing during the summer months has a long history in Norway. Traditionally this has taken place in mountain grazing areas marginal for other uses, thus constituting an important component of local farm economies and a basis for the much of the Norwegian settlement pattern. When Norway was colonized after the last ice age, this was done by wild flora and fauna, humans and domesticated animals in concert, in such a fashion that human activity has facilitated suitable habitat for many species. Thus, human activity, rather than being an exclusive threat to biodiversity, also forms a basis for the existence of a whole suite of ecosystem types, collectively known as the cultural landscape. It is fair to denounce mountain grazing areas as semi-cultural, while the actual proportion of wilderness in Norway (as by definition) is ~5% (DN-rapport, 6-1995). The realization of this is followed by the coming trend that nature conservation is not just something that can be limited to protected areas, but rather should be tightly connected with how we interact with our surroundings, including resource management. The managed use of mountain grazing areas in Norway has been documented as far back as the Bronze Age (Kvamme, 1988; Moe et al., 1988). Also, mountain areas in Norway and much of Europe have an evolutionary history of natural grazing (Körner, 2003), thus the ecosystems in question have been shaped by the long term interplay between natural elements and domesticated/semi-domesticated livestock. But the presence of large herbivores, in numbers and species composition (grazing habits), has been highly varying in time and space, and the actual grazing history of a locality is likely to be important for the effects of future grazing (Augustine & McNaughton, 1998). 2 As with agricultural practices in general, the use of mountain areas for livestock grazing is affected by trends in agriculture and incentives given by political guidelines. The introduction of chemical fertilizer and substantial use of subsidized feed concentrate are relevant examples as well as large carnivore policies. Norwegian farms have, since the 1970's, been shut down at an alarming rate, and while the affected farmed areas have been mostly upheld by fusion into bigger units, this is now less and less the case. Many marginal areas have been abandoned already, and succession is evident throughout the country. The practice of mountain field grazing though, show less decline; but inaccessible areas are likely to be dismissed if more productive lowland areas become available. Some localities though have been subject to intensified grazing regimes in recent times. Also, the overall composition of livestock used in mountain grazing has changed radically since the middle of the 20. century; away from a diverse crowd of sheep, goats, cattle and horses to now almost exclusively sheep. Large herbivores affect ecosystems by grazing, trampling and defecation , and plant communities show differentiated responses to different grazing intensities (Augustine & McNaughton, 1998; Mysterud & Austrheim, 2005). Therefore, it is of prime importance to investigate the effects different grazing practices, and changes in these, might have on the biological composition of mountain grazing areas, as it is of general interest that these areas can sustain support for future grazing and/or other primary production goals (including berry harvesting) and maintain biodiversity at local and regional scales, as well as provide aesthetic qualities and recreational use. But why focus on farming and livestock grazing in a pollinator context? A substantial amount (~44%) of the norwegian red list of species is made up by species dependent on the open cultural landscape (Kålås et al., 2010), which is highly dependent on these managed practices. A fair proportion of these species can be assumed to be important pollinators, as flowering plant abundance has been plunging. Of a total of 207 wild bee species in Norway, about one in three is on the red list, while 12 are considered extinct (Artsdatabanken, 2015b). While mountaneous heathlands are still mostly intact, and by that its pollinator communities, changes in livestock management might influence this for better or worse. In addition, plant community composition is under the influence of (and increasingly affected by), other external factors like accelerating climate change and long transport atmospheric pollution. In conclusion, relevant management recommendations, as guided by several objective and subjective 3 aspects and goals, must be informed by consideration of several external factors co-influencing on the whole of this nature system. One important such aspect is the health of the pollinator community. 1.2 The grazing experiment at Hol In 2001, a permanent landscape scale grazing experiment was established at Hol municipality, Buskerud county, south Norway (between 60°40' – 60°45' N and 7°55' – 8°00' E). The 2.7 km2 field area (Figure 1a) is situated in a south facing hillside, ranging from ~1100 to ~1300m a.s.l; the core of the area can be classified as lower alpine climatic zone, while bordering the tree-line ecotone at its lower parts. It consists of a fine scale mosaic of distinct vegetation types of varying grazing value, including moss- and grass snow-beds, lichen heath and dwarf shrub heath, low herb- and tall forb meadow, bog and fen as the most important in this setting. The site is, by its geological and climatic conditions and relatively moderate productivity, representative for much of the mountain grazing areas of southern Norway. Prior to the experiment, the tree line had been steadily advancing into the lower parts of the field area. The field was fenced and subdivided into nine (A - I) adjoining areas (from now on called sub- enclosures), each approximately 300 m wide and 1 km deep, such that each sub-enclosure spans most of the elevation gradient. Before grazing started, vegetation type mapping was done at the resolution scale of 2 ha (Figure1b), from which grazing capacity was estimated, and importantly, all the relevant vegetation types are present in similar amounts in the nine sub-enclosures (Rekdal, 2001). Each of the nine sub-enclosures has since 2002 been permanently subjected to either of three treatments of sheep grazing pressure: grazing-ceased , grazing-maintained (25 sheep/km2) , grazing-increased (80 sheep/km2). Although actual sheep densities were adjusted to vegetation type estimates, i.e. bare rock was subtracted from the area, and vegetation types were differentially weighted for grazing quality. The grazing-maintained treatment represent the historic grazing intensity at the site, which is thought to

Load more